1. Field of the Invention
The present invention is generally related to light emitting diodes and, more specifically to a light emitting diode structure which eliminates certain undesired modes so that the generation of light in desired modes is enhanced.
2. Description of the Prior Art
Many different types of light emitting diodes are known to those skilled in the art. For example, U.S. Pat. No. 3,293,517, which issued to Biard et al on Dec. 20, 1966, discloses a gallium-arsenide radiant diode which emits light in the near infrared spectrum when the junction of the diode is forward biased.
In the years since the initial development of the light emitting diode, or LED, many attempts have been made to improve its generally low overall efficiency. Because the emission of light within a light emitting diode is isotropic, a significant portion of the light emitted within the active region of the diode is emitted in disadvantageous directions toward the internal portions of the diode and is therefore ineffective in supplementing the total effective light output of the diode. In addition, a portion of the light which is emitted in the general direction toward the output surface of the diode, but not within the critical angle for reflection, passes in a direction which is reflected back into the body of the diode because of the relative indices of refraction of the diode structure and the surrounding atmosphere. It has been determined that, in a typical light emitting diode, approximately 2% of the total amount of light generated in the active region of the diode is actually available as light output from a single surface of the diode for each reflective pass. As a result of this low external efficiency of the typical light emitting diode, many efforts have been made since the initial development of the light emitting diode to improve its output efficiency.
U.S. Pat. No. 4,675,058, which issued to Plaster on Jun. 23, 1987, discloses a method of manufacturing a high bandwidth, high radiance, surface emitting LED. An LED made in accordance with this patent comprises an epitaxial layer of semiconductive material which is formed in a mesa shape.
U.S. patent application Ser. No. 07/764,651, which was filed on Sep. 24, 1991 and assigned to the Assignee of the present application, describes a high speed visible light emitting diode which emits visible light in a highly efficient manner to achieve an improved intensity of light emission compared to diodes previously known to those skilled in the art. The LED can utilize a substrate having a mesa region to provide an elevated surface. It is provided with a blocking layer on the substrate with a lower confining layer on the blocking layer. An active layer is provided on the lower confining layer with an upper confining layer above the active layer.
Other attempts to improve the efficiency of the light emitting diode output include the development of etched-well LED's that improve the coupling of radiation from the LED to an optical fiber. Additional techniques include the use of a "pigtailed" optical source having a short piece of optical fiber permanently attached to an LED package to provide a high performance optical link. Some techniques involve the use of a microlens associated with the active region of an LED to direct the emitted light in a preferred direction. These and other related concepts are discussed in an article by R. Stephen Speer and Bobby M. Hawkins titled "Planar--Heterostructure GaAlAs LED's Packaged for Fiber Optics" which was published in the IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. CHMT-3, Number 4, December 1980.
The work described above primarily involves efforts to improve the output of light emitting diodes. However, it should be understood that these and other techniques could alternatively be applied to lasing devices such as laser diodes. Furthermore, it should also be understood that the basic concept of limiting light output to a single mode is generally known to those skilled in the art in relation to fiber amplifiers. For example, the gain and optimal length in the Erbium-doped fiber amplifiers with 1480 nm pumping in the IEEE Photonics Technology Letters, Volume 4, number 4, April 1992 by Min-Chuan Lin and Sien Chi, provides approximate analytic expressions for the gain and optimal length of the 1480 nm-pumped Erbium-doped fiber amplifier with an arbitrary dopant distribution. The numerical calculations show that the values predicted from the analytic expressions of the maximal gain and optical fiber length as functions of input signal and pumped powers are very accurate for narrow dopant distributions confined to the center of the fiber core.
This article describes single-mode fibers which, as will be discussed below, provide an optical fiber within a cladding layer in which the diameter of the optical fiber is particularly selected to inhibit emission of light in directions other than generally parallel to the central axis of the fiber core. Most of the techniques currently applied by those skilled in the art to improve the output of light emitting diodes or lasers are either related to increasing the total production of light from the diode or to more efficiently handling the output of the light emitting diode. For example, steps such as those discussed in the Plaster patent described above increase the radiance of the diode.
The majority of the techniques known to those skilled in the art for the improvement of the output efficiency of a light emitting diode attempt either to increase the overall intensity of light emitted by the diode, to improve the handling efficiency of the light emitted toward the output surface of a light emitting diode or to reflect light from a disadvantageous direction toward an advantageous direction within the structure of the diode. It would therefore be significantly beneficial to the field of light emitting devices if an LED could be developed which actually inhibits the creation of light in disadvantageous directions so that the overall emission of light in advantageous directions is correspondingly enhanced.